The great skua is an aerial apex predator, both preying on other seabirds and bullying them for their catches.[7]

Apex predators affect prey species' population dynamics and populations of other predators, both in aquatic and in terrestrial ecosystems. Non-native predatory fish, for instance, have sometimes devastated formerly dominant predators. A lake manipulation study found that when the non-native smallmouth bass was removed, lake trout, the suppressed native apex predator, diversified its prey selection and increased its trophic level.[8] As a terrestrial example, the badger, an apex predator, predates on and also competes with the hedgehog, a mesopredator, for food such as insects, small mammals, reptiles, amphibians and ground-nesting bird's eggs. Removal of badgers (in a trial investigating bovine tuberculosis) caused hedgehog densities to more than double.[9] Predators that exert a top-down control on organisms in their community are often considered keystone species.[10] The presence of any top predator (bear, big cat, wolf, dog, or especially human) can greatly alter the behavior of such a smaller predator as a badger.[11]

Apex predators can have profound effects on ecosystems, as the consequences of both controlling prey density and restricting smaller predators, and may be capable of self-regulation.[12] They are central to the functioning of ecosystems, the regulation of disease, and the maintenance of biodiversity.[13] When introduced to subarctic islands, for example, Arctic foxes' predation of seabirds has been shown to turn grassland into tundra.[14] Such wide-ranging effects on lower levels of an ecosystem are termed trophic cascades. The removal of top-level predators, often through human agency, can cause or disrupt trophic cascades.[15][16][17] For example, reduction in the population of sperm whales, apex predators with a fractional trophic level of 4.7, by hunting has caused an increase in the population of large squid, trophic level over 4 (carnivores that eat other carnivores).[18] This effect, called mesopredator release,[19] occurs in terrestrial and marine ecosystems; for instance, in North America, the ranges of all apex carnivores have contracted whereas those of 60% of mesopredators have grown in the past two centuries.[20]

Because apex predators have powerful effects on other predators, on herbivores, and on plants, they can be important in nature conservation.[21] Humans have hunted many apex predators close to extinction, but in some parts of the world these predators are now returning.[22] They are increasingly threatened by climate change. For example, the polar bear requires extensive areas of sea ice to hunt its prey, typically seals, but climate change is shrinking the sea ice of the Arctic, forcing polar bears to fast on land for increasingly long periods.[23]

Ecologists have debated whether humans are apex predators. Sylvain Bonhommeau and colleagues argued in 2013 that across the global food web, a fractional human trophic level (HTL) can be calculated as the mean trophic level of every species in the human diet, weighted by the proportion which that species forms in the diet. This analysis gives an average HTL of 2.21, varying between 2.04 (for Burundi, with a 96.7% plant-based diet) and 2.57 (for Iceland, with 50% meat and fish, 50% plants). These values are comparable to those of non-apex predators like anchovy or pig.[29] Peter D. Roopnarine criticised Bonhommeau's approach, arguing that humans are apex predators, and that the HTL was based on terrestrial farming where indeed humans have a low trophic level, mainly eating producers (crop plants at level 1) or primary consumers (herbivores at level 2), which as expected places humans at a level slightly above 2. Roopnarine instead calculated the position of humans in two marine ecosystems, a Caribbean coral reef and the Benguela system near South Africa. In these systems, humans mainly eat predatory fish and have a fractional trophic level of 4.65 and 4.5 respectively, which in Roopnarine's view makes those humans apex predators.[b][30]

Apex predators are thought to have existed since at least the Cambrian period, around 500 million years ago. Extinct species cannot be directly determined to be apex predators as their behaviour cannot be observed, and clues to ecological relationships, such as bite marks on bones or shells, do not form a complete picture. However, indirect evidence such as the absence of any discernible predator in an environment is suggestive. Anomalocaris was an aquatic apex predator, in the Cambrian. Its mouthparts are clearly predatory, and there were no larger animals in the seas at that time.[31]

Tiger sharks are popular ecotourism subjects, but their ecosystems may be affected by the food provided to attract them.

Ecotourism sometimes relies on apex predators to attract business.[43][44] Tour operators may in consequence decide to intervene in ecosystems, for example by providing food to attract predators to areas that can conveniently be visited.[43] This in turn can have effects on predator population and therefore on the wider ecosystem.[43] As a result, provisioning of species such as the tiger shark is controversial, but its effects are not well established by empirical evidence.[43] Other affected apex predators include big cats and crocodiles.[44]

The reintroduction of predators like the lynx is attractive to conservationists, but alarming to farmers.

In some densely populated areas like the British Isles, all the large native predators like the wolf, bear, wolverine and lynx have become locally extinct, allowing herbivores such as deer to multiply unchecked except by hunting.[45] In 2015, plans were made to reintroduce lynx to the counties of Norfolk, Cumbria, and Northumberland in England, and Aberdeenshire in Scotland as part of the rewilding movement.[46] The reintroduction of large predators is controversial, not least because of concern among farmers for their livestock.[46] Conservationists such as Paul Lister propose instead to allow wolves and bears to hunt their prey in a "managed environment" on large fenced reserves.[46]

^Zoologists generally exclude parasites from trophic levels as they are (often much) smaller than their hosts, and individual species with multiple hosts at different life-cycle stages would occupy multiple levels. Otherwise they would often be at the top level, above apex predators.[4]

^However, humans had a network trophic level (NTL) of 4.27 in the coral reef system, compared to an NTL of 4.8 for the blacktip shark in the same system. Therefore, humans were not the topmost apex predator there.[30]

^Its trophic level would be exactly 4 if the fish was a pure herbivore, higher if the fish was itself carnivorous.

^Jones, Lucy. "The rewilding plan that would return Britain to nature". BBC. Retrieved 6 June 2018. wolves, bears and lynx roamed the land. ... Humans chopped down the trees to make space for farms, and hunted the large animals to extinction, leaving plant-eaters to decimate the country's flora. Britain is now one of the few countries in the world that doesn't have top predators.